Toughened bismaleimide resin systems

ABSTRACT

Toughened heat-curable bismaleimide resin systems which contain alkenylphenoxy-terminated polysiloxane modifiers and compatibilizing agents. Cured resin systems exhibit increased toughness without a concomitant decrease in thermal stability at elevated temperatures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention relates to high performance resin systemscontaining bismaleimides. More particularly, the subject inventionrelates to toughened bismaleimide resin systems prepared by addingtougheners which are alkenylphenoxy terminated polysiloxanes. Thetoughened resins find use as matrix resins in fiber-reinforced prepregsand composites, and as structural adhesives.

2. Description of the Related Art

Bismaleimide resin systems are noted for their high strength, hightemperature performance, particularly as matrix resins infiber-reinforced prepregs and composites. Unfortunately, bismaleimidestend to be somewhat brittle, and thus subject to impact-induced damage.For the same reasons, structural adhesives based on bismaleimides havenot achieved the wide use attributable to more flexible resins such asthe epoxies.

In the past, efforts have been made to toughen bismaleimides, forexample, by the addition of acrylonitrile/butadiene elastomers to theresin system. Unfortunately, the degree of toughening available by thismethod is less than desirable. Moreover, a loss of heat resistance isseen as the amount of elastomeric toughener increases. Finally, theamount of toughener added is limited due to the formation of multiphasesystems. Modifying bismaleimide systems with long chain, flexible epoxyresins has not proven successful for somewhat the same reason. Onlyminor amounts of epoxy resins are generally compatible with bismaleimideformulations.

The addition of alkenylphenols such as 2-allylphenol and2-propenylphenol and their multiring homologues such as2,2'-diallylbisphenol A has increased the toughness of bismaleimideresin systems, but once again, the degree of toughness obtained is lessthan that ultimately desirable. It would be desirable to prepare atoughener for bismaleimide resin systems which is compatible with otherresin components, which forms a homogenous resin system and which doesnot cause a loss of heat resistance.

SUMMARY OF THE INVENTION

It has now been surprisingly discovered that polysiloxanes having theirtermini capped with alkenylphenols are compatible with bismaleimideresins and can be used in appreciable amounts to toughen such resins.The toughened systems which result maintain a high degree of thermalstability at elevated temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The alkenylphenoxy-terminated, polysiloxane modifiers of the subjectinvention are prepared through the reaction of an epoxy-functionalpolysiloxane with an alkenylphenol, generally in the presence of acatalyst which is effective in promoting the reaction of epoxy groupswith phenolic hydroxy groups.

In the case of a difunctional polysiloxane, the general reaction may beillustrated as follows: ##STR1## wherein each R may be individuallyselected from the group consisting of substituted and unsubstituted C₁-C₆ lower alkyl, C₁ -C₆ lower alkoxy, aryl, acetoxy and cycloalkyl, andwherein R¹ is or C₃ -C₆ alkenyl radical such as an allyl, or propenylradical.

The epoxy-functional polysiloxanes are prepared by known methods.Preferably, these polysiloxanes are prepared through the equilibriumpolymerization of the readily availablebis(3-glycidoxypropyl)tetramethyldisiloxane with one or more cyclicpersubsituted siloxanes, preferably octamethylcyclotetrasiloxane andoctaphenylcyclotetrasiloxane. However other starting materials may beused. It is also possible to derivatize a silane-functional polysiloxanewith an epoxy-group containing compound such as 3-glycidoxyallene.

The equilibrium polymerization is catalyzed by known catalysts such asthe tetraalkylammonium salts, particularly tetramethyl andtetrabutylammonium hydroxide, and the tetraalkylammonium siloxanolates,particularly tetramethyl and tetrabutylammonium siloxanolates. Forfurther information relative to epoxy-functional polysiloxanes, see J.Riffle, et. al. Epoxy Resin Chemistry II. pp. 24-25, Bauer, Ed. ACSSymposium Series No. 221, American Chemical Society.

The alkenylphenols utilized in preparing the alkenylphenoxy terminatedpolysiloxanes of the subject invention are commercially available from anumber of sources. The alkenylphenols may be ortho or para-alkenylmononuclear phenols or ortho-alkenyl polynuclear phenols, wherein thealkenyl group contains from 3 to about 6 carbon atoms. Examples ofmononuclear alkenylphenols are 2- and 4-allylphenol, 2- and4-propenylphenol, 2,5-diallylhydroquinone, and the like. Examples ofpolynuclear alkenylphenols include 2,2'-dialkenylbisphenols such as2,2'-diallylbisphenol A, 2,2'-diallylbisphenol F and2,2'-diallylbisphenol S and the corresponding dipropenyl compounds, andoligomeric polynuclear alkenylphenols corresponding to the formula:##STR2## wherein A is selected from the group consisting of ##STR3##wherein each R² may individually be aryl, cycloalkyl, or alkyl; ##STR4##wherein o is an integer from 0 to about 2, and where p is an integerhaving values of from 1 to about 10 and wherein R¹ is a C₃ -C₆ alkenylgroup such as a substituted or unsubstituted allyl or propenyl group.

Such alkenylphenols may be readily prepared by known methods, forexample by reacting the analogous phenol with allyl chloride or allylbromide to form the allyloxy ether followed by a Claisen rearrangementto the allylphenol. The allylphenols may be easily rearranged to thecorresponding propenylphenols by isomerization in alkali, for example bythe process disclosed in J. Am. Chem. Soc. (1956) pp 1709-13.

The oligomeric alkenylphenols may be prepared by reaction of an excessof a diphenol or its alkali metal salt with an activated dihalobenzenoidcompound, for example by the process disclosed in U.S. Pat. No.4,175,175. Following formation of the phenol terminated oligomer, theallyl or propenyl ether is prepared as disclosed previously, followed bya Claisen rearrangement to the ortho-substituted alkenylphenol.

The reaction of the alkenylphenol with the epoxy-functional polysiloxanegenerally takes place at elevated temperatures, for example from80°-200° C., preferably from 100°-150° C. The reaction generallyrequires catalysis by catalysts which promote the reaction betweenepoxies and phenols in a selective manner. Suitable catalysts are, forexample tris-substituted phosphines and phosphonium salts, particularlytriphenylphosphine, 2,5-dihydroxyphenylphosphonium hydroxide innersalts, phosphoranylidene succinic acid derivatives, andphosphoranylidene maleimide derivatives. Completion of the reaction maybe determined by measuring epoxy equivalent weights by titration.

Preferred alkenylphenoxy terminated polysiloxanes are those preparedfrom 2-allylphenol, 2-propenylphenol, 2,2'-diallylbisphenol A,2,2'-diallylbishenol F, 2,2'-diallylbisphenol S or their corresponding2,2'-dipropenyl analogs and bis (3-glycidoxypropyl)-polysiloxanepolymers wherein the silicon atoms are disubstituted with methyl orphenyl groups. While the preferred modifiers are based on difunctionalepoxy-terminated polysiloxanes because of their ready availability,modifiers based on polyfunctional epoxy-terminated or epoxy-substitutedpolysiloxanes are also possible and may be desirable in certain resinsystems. The terms alkenylphenoxy-terminated polysiloxanes andepoxy-terminated polysiloxanes should be taken as including thecorresponding polyfunctional siloxanes and their alkenylphenoxyderivatives as well. Because of their desirable properties and readyavailability, the 2-, and 4-alkenylphenoxy-terminated linearpolysiloxanes are preferred. Polysiloxanes derived from 3-alkenylphenolsare not contemplated by the subject invention.

The use of the subject invention alkenylphenoxy-terminated polysiloxanemodifiers in bismaleimide resin systems is generally accomplished bypretreatment of the bismaleimide with the alkenylphenoxy terminatedpolysiloxane modifier in the absence of catalyst in order to assure auniform distribution of the modifier in the matrix resin. Pretreatmentgenerally occurs over about the same temperature range used to preparethe modifier. Bismaleimide resin systems which are useful are well knownto those skilled in the art. Particularly preferred bismaleimides arethe commercially available eutectic bismaleimides which have relativelylow softening points, such as Compimide® 353 available fromBoots-Technochemie.

These and other eutectic bismaleimides rely on the phenomenon wherebythe melting point of a mixture of components is lower than that of theindividual components themselves. Compimide® 353, for example, containsthree bismaleimides, the bismaleimides of 4,4'-diaminodiphenylmethane,1,6-diamino-2,2,4-trimethylhexane, and 2,4-diaminotoluene.

The bismaleimides are preferably co-cured with a correactant which is analkenylphenol or alkenyl-substituted aryl compound. Preferredalkenylphenol coreactants are 2,2'-diallyl and 2,2'-dipropenylbisphenolA, 4,4'-bis(2-propenylphenoxy)benzophenone and4,4'-bis(2-propenylphenoxy)diphenylsulfone. Preferredalkenyl-substituted aryl compounds are, for example, divinylbenzene andm-diisopropenylbenzene.

Epoxy resins may also be used as components of the toughenedbismaleimide resin systems of the subject invention. These resins arewell known to those skilled in the art and are described, for example,in the treatise Handbook of Epoxy Resins McGraw-Hill, ©1967. When epoxyresins are utilized, curing agents are generally utilized also. Suchcuring agents may be of the amine, anhydride, or phenolic type. Curingagents are described in the Handbook of Epoxy Resins in chapters 6-12.

Preferred epoxy resins are the glycidyl ethers of the bisphenols andtris(4-hydroxyphenyl)methane, and the analogous glycidyl derivatives ofamines and aminophenols, particularly p-aminophenol, methylenedianiline,and 4,4'-diaminodiphenylsulfone. The glycidyl ethers of novolak resins,such as those based on phenol-formaldehyde and cresol-formaldehydeadducts, and the phenol or cresol derivatized di- andpolycyclopentadienes are also well suited. The preferred curing agent is4,4'-diaminodiphenylsulfone

Cured products containing the alkenylphenoxyterminated polysiloxanemodifiers of the subject invention exhibit improved mechanicalproperties without sacrificing thermal stability. Thealkenylphenoxy-terminated polysiloxane modifiers also find use asprecursors for radiation and free radical vulcanizable systems.

EXAMPLE 1 Preparation of Tetramethylammonium Siloxanolate

Into a 250 ml three-necked round-bottomed flask fitted with a mechanicalstirrer and reflux condenser are placed octamethylcyclotetrasiloxane(118.6 g, 0.4 mol) and tetramethylammonium hydroxide pentahydrate (18.1g, 0.1 mol). The mixture is stirred at 70° C. for 48 hr. under N₂ flow,adjusted to be sufficient to dehydrate the system. The resulting viscoussyrup is used as a polymerization catalyst without further purification.

EXAMPLE 2 Preparation of an Epoxy Terminated DiphenylDimethypolysiloxaneCopolymer

Octamethylcyclotetrasiloxane (534.4 g), Octaphenylcyclotetrasiloxane(534.4 g), bis (3-glycidoxypropyl)tetramethyldisiloxane (90.7 g), andtetramethylammonium siloxanolate (12.0 g) are charged to a 2.0 literthree necked round-bottomed flask equipped with a mechanical stirrer,reflux condenser, and nitrogen inlet. The resulting mixture is stirredat 80° C. for 48 hours under N₂. During this period of time, theviscosity of the reaction mixture is observed to increase and to thenreach a stable value. Subsequently, the temperature of the mixture israised to 150° C. for 4 hours to effect destruction of the catalyst. Thereaction mixture is cooled to room temperature and filtered. The crudeoligomer is purified by extracting the equilibrium cyclics with methanol(300 ml×2). After evaporation of methanol, the oligomer is further driedunder mechanical pump vacuum (1 torr) at 150° C. The purified oligomeris a colorless, viscous oil (1100 g) having an epoxy equivalent weight(EEW) of 1210.

EXAMPLE 3 Preparation of an Epoxy Terminated Dimethylpolysiloxane

Using the general procedure described in Example 2, a reactor is chargedwith bis(3-glycidoxypropyl)tetramethyldisiloxane (18.3 g),octamethylcyclotetrasiloxane (182.0 g), and tetramethylammoniumsiloxanolate (1.4 g). The resulting mixture is heated to 80° C. for 48hours and 50° C. for 4 hours under N₂. The reaction mixture is cooled toroom temperature and filtered. Removal of low boiling fractions at 150°C. and 1 torr gives a colorless, viscous oil (180 g, EEW=2200).

EXAMPLE 4 Preparation of a 2-Allylphenoxy-TerminatedDiphenylDimethylpolysiloxane

A 250 ml glass reactor is charged with the epoxy terminated siloxanefrom Example 2 (32.4 g), 2-allylphenol (5.2 g), and triphenylphosphine(0.07 g). The resulting mixture is heated to 130° C. for 12 hours. Atthe end of this time, the EEW of the mixture determined by titrationshows complete consumption of the epoxy groups. Removal of excess2-allylphenol under vacuum gives a 2-allylphenoxy-terminated siloxanecopolymer as an odorless, clear oil.

EXAMPLE 5 Preparation of a 2-Propenylphenoxy Terminated Polysiloxane

A mixture of the epoxy terminated polysiloxane from Example 2 (32.4 g),2-propenylphenol (5.2 g), and triphenylphosphine (0.07 g) is heated inthe same manner as described in Example 4. Removal of excess2-propenylphenol under vacuum gives a 2-propenylphenoxy-terminatedsiloxane copolymer as an odorless, clear oil.

EXAMPLES 6,7 Preparation of Heat-Curable Resin Compositions

The allylphenoxy-terminated polysiloxane copolymer (17.2 g) from Example4 is treated with Compimide® 353 (Boots-Technochemie, 10 g) at 145° C.for 4 hours under N₂. To the pretreated mixture, additional Compimide®353 (15 g), a glicidyl ether of tris (4-hydroxyphenyl)methane (Tactix®742, Dow Chemical Co., 36 g), and a glicidyl ether of bisphenol A(DER®332, Dow Chemical Co., 12 g) are added. The resulting mixture isstirred at 130° C. for 30 minutes. At 70° C., 3,3'diaminodiphenylsulfone (22 g), 4,4'-(p-aminophenoxy)diphenylsulfone (4.0g), and a fumed silica (CAB-O-SIL, M-5, 3.6 g) are added while stirring.The resulting substrate is coated onto a 112 glass fabric. Similarly, aformulation is made with the propenylphenoxy-terminated polysiloxanecopolymer from Example 5. In this case, pretreatment of the modifier wasaccomplished by heating at 135° C. for 3 hours. Aluminum single lapshear strengths are measured by following the method described in ASTMD-1002. Results are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Lap shear strengths of cured resin compositions                               formulated with siloxane modifiers.                                                          Lap Shear Strength (psi)                                       Siloxane Modifier                                                                              20° C.                                                                           205° C.                                     ______________________________________                                        from Example 4   1700      2600                                               from Example 5   2000      2400                                               ______________________________________                                         Cure:                                                                         177° C./4 Hours + 220° C./2 Hours + 250° C./1 Hour  

EXAMPLE 8

A propenylphenoxy-terminated polysiloxane copolymer (8.3 g) from Example5 is treated with Compimide® 353 (4.5 g) at 135° C. for 3 hours. To theabove mixture, the bismaleimide of 4,4'-diaminodiphenylmethane (5.0 g),a glycidyl ether of 9,9'-di(4-hydroxyphenyl)fluorene (17 g), and aglycidyl ether of bisphenol F (XU® 3336, CIBA-GEIGY. 6.0 g) are added.The resulting mixture is homogenized at 130° F. for 30 minutes. Afteraddition of 4,4'-diaminodiphenylsulfone (8.0 g) and2-Ethyl-4-methylimidazole (0.05 g) at 70° C., the final resin mixture iscoated onto a 112 glass fabric. The adhesive is cured by heating for 4hours at 177° C., 2 hours at 220° C. and 1 hour at 250° C. The singlelap shear strengths (alumimum) are 2730 psi at 20° C. and 3230 psi at205° C., respectively.

EXAMPLE 9

A reactor is charged with the epoxy-terminated dimethylsiloxane fromExample 3 (22 g), 2-propenylphenol (2.0 g), and triphenylphosphine (0.04g). The resulting mixture is heated to 135° C. for 10 hours under N₂.After confirming the completion of the reaction by titration, adiglycidyl ether of bisphenol F (1.0 g) and Compimide® 353 (6.0 g) areintroduced. The resulting mixture is heated to 135° C. for 3 hours toobtain an opaque mixture. The above mixture (10.0 g) anddimethylbenzylamine (0.01 mg) are mixed in an aluminum dish and cured at177° C. for 2 hours and 200° C. for 10 hours. The cured elastomerexhibits improved strength as compared to siloxane homopolymers.Thermogravimetric analysis (TGA) data of the elastomer are shown inTable II.

                  TABLE II                                                        ______________________________________                                        Substrate     5% Wt Loss 10% Wt Loss                                          ______________________________________                                        Silicone-BMI  400° C.                                                                           430° C.                                       ______________________________________                                    

The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:
 1. A toughened bismaleimideresin system, comprising:(a) a bismaleimide resin, and (b) an effectiveamount of a toughener comprising an alkenylphenoxy-terminatedpolysiloxane, wherein the alkenyl group contains from 3 to about 6carbon atoms.
 2. The system of claim 1 wherein saidalkenylphenoxy-terminated polysiloxane is an allylphenoxy orpropenylphenoxy-terminated polysiloxane.
 3. The system of claim 2wherein the silicon atoms of said polysiloxane are disubstituted withmethyl radicals, phenyl radicals, or mixtures thereof.